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Kotlin compile works, mvn compile and intellij run-app does not

I'm having a very odd issue which I cannot explain.
I've made an artifact using gitlab ci, which I'm now importing as a dependency in another project using maven.
When I import this into the new project, I get some very strange behaviour in regards to the classes in the imported dependency. These behaviours are as follows:
In Java files (Test.java), the package is found and Intellij reports no issues. I can even navigate to the source code in the library.
In Kotlin files, Intellij complains that this package cannot be found. I can't navigate to the source code as the IDE says "unresolved reference". If I do a maven artifact search, it is found but adding it makes no difference as the dependency is already in my project.
maven compile/package also complains that this package cannot be found
The dependency is definitely present:
I've tried the following:
Reimporting maven projects
Invalidating caches
Changing java versions, 8 - 11
The artifact is written in java 11 if that makes any difference.
Please help, this is driving me crazy.

Thanks to those that answered!
After a long 24 hours of debugging, it turns out the issue is related to how intellij & maven look for packages in dependencies.
I was compiling with the spring-boot-maven-plugin which was building the jar with a top BOOT-INF directory, and the package structure below this.
This meant both intellij and maven were confused as they seemed to be able to find the package but not build it.
I've since removed the plugin and left now just compile with the kotlin-maven-plugin which has the package structure at the top level of the jar.
Now when I import into a project, the project can find and compile this dependency.

Eclipse module problems jdk 10

I'm having a new problem this morning with my project (it worked fine on Friday). Eclipse seems to be having trouble figuring out it's module / class path, and nothing I'm trying seems to affect it. I'm using the following Eclipse configuration:
Version: 2019-03 M1 (4.11.0 M1)
Build id: 20190117-2133
Oracle JDK 10.0.2
My project is a Java Maven multi-module (those are maven modules, not java modules) project, but does not define any java modules itself (we haven't converted it yet). When attempting to compile it, eclipse throws up its hands and sends the message
"The package java.sql is accessible from more than one module: <unnamed>, java.sql".
While, on the surface, this seems helpful, I cannot find another module that defines the java.sql package, and the only instance of a class from the package (e.g.: java.sql.ResultSet) exists within the JDK system supplied library. This also happens with the javax.xml package with the same error.
I've attempted closing and re-opening eclipse, performing a clean, and a maven update project for everything, to no avail. I've inspected the eclipse .log file, and there's nothing useful in there. Is there anyway to figure out what other module Eclipse thinks it's pulling that package from?
My next step is to delete the entire project and re-import it.
UPDATE: I think I may have found the offender... there's a dependency that declares a couple of classes into the java.sql package, and that's probably borking things up. The specific JAR is net.sourceforge.hatbox:hatbox:1.0.b8, and is a transitive dependency of the com.h2database:h2:1.4.197 jar. Is there any way to tell eclipse to ignore the package from that JAR?
Update 2: Build Path:

I'm not sure I count this as a true solution, but I was able to find a workaround to my problem. I downgraded eclipse from the installed version listed in the question (4.11.0) to the base photon install (4.8.0), and I can now compile and work again.

IntelliJ is unable to import hamcrest library

I'm having some trouble importing my project from build.gradle file.
It's not able to find classes from hamcrest-core-1.3.jar.
gradle clean build
runs successfully on terminal.
My environment is
Intellij 2016.3.1
Gradle 2.14.1
It was able to resolve the reference one time but it went away on restart. When it was able to resolve hamcrest, it later failed to resolve pigunit jar. I got NoSuchMethodError (class was loaded from wrong jar) also once.
Invalidate Cache also didn't work.
I tried importing the project from scratch multiple times.
Please let me know if any other information is needed.
For some reason org.hamcrest.CoreMatchers loads CoreMatchers from
mockito-all-1.10.19.jar instead of hamcrest-core-1.3.jar.I played around with the order of these two jars in the modules libraries.
Please refer to the screenshot below.

How do I get IntelliJ to recognise resource bundle when compiling?

Ok, first up I am almost completely unfamiliar with Java and Java tooling.
I am attempting to contribute a bugfix to a Jenkins plugin. Code-wise the fix is simple, my problem is in trying to compile the project and run the tests locally.
I have installed IntelliJ IDEA 15CE and started a new project, pointing it to my local clone of the plugin repo.
It has gone away and 'downloaded the internet' in terms of Maven dependencies. Mostly everything looks ok. What I want to do is to run the tests...
If I right-click at the root of the project and select 'Run All Tests' I get the error:
Error:(115, 29) java: cannot find symbol
symbol: variable Messages
location: class hudson.plugins.copyartifact.BuildSelector
The source of this error is a line:
console.println(Messages.CopyArtifact_MissingSrcArtifacts(srcDir));
https://github.com/jenkinsci/copyartifact-plugin/blob/master/src/main/java/hudson/plugins/copyartifact/BuildSelector.java#L115
As far as I can tell the Messages symbol should resolve to this properties file:
https://github.com/jenkinsci/copyartifact-plugin/blob/master/src/main/resources/hudson/plugins/copyartifact/Messages.properties
...which IDEA recognises as a 'Resource Bundle' in the project tree view.
But it doesn't seem to recognise it when compiling the project.
I have no idea where to start fixing this. The tests run fine if I push a commit to a pull request on GitHub and let the cloudbees.com Jenkins server run them, but I'd like to be able to test stuff locally first.

Run the maven compile command from Intelli-J Maven tool. After compiling error will vanish.
When you compile it will generate a Messages.java file in generated sources automatically.

Ho do I generate eclipse project without compiling or ignoring compilation errors using SBT and play framework?

Recently our project's dependencies artifact name has been changed so as the package names and some classes' names. Now we need to update our project dependencies for our next release. Problem is when invoking eclipse from play console it just doesn't do that if there's any compilation errors which of course is expected. I just want to update the eclipse project settings and fix compilation problems later.

It should generate the projects even if there are compilation problems. You might want to check your sbteclipse version.
There might be a chance that it will not even compile when generating the eclipse files. This could theoretically be triggered by a related sbt issue: compile is executed for discoveredSbtPlugins when the project is not a plugin
So if you are not creating an sbt plugin and have an updated sbteclipse version and still have problems, you could try setting: discoveredSbtPlugins := PluginDiscovery.emptyDiscoveredNames.
I would strongly advise you to first see if the compilation problems are causing the projects not to be created. In the past I have often created projects when I had compiler errors. It will for example not create the project if you have an unresolved dependency.